Hairspring for balance wheel hairspring resonator and production method thereof

ABSTRACT

The spiral includes turns of rectangular section, whose pitch p and/or thickness e can vary from the inside curve towards the outside curve, or whose winding can deviate from the line of a perfect spiral. The inside curve can also be extended by a self-locking washer for fixing the spiral on the balance arbour with no play. The spiral is manufactured by photolithography and galvanic growth, or by micro-machining an amorphous or crystalline material such as a silicon wafer.

The present invention concerns a flat resonator spiral for a sprungbalance obtained by a manufacturing method for improving isochronism byacting, on the one hand, on construction parameters of the spiral assuch, and on the other hand, on a mode of securing it to the balancearbour for reducing the geometrical deviation inherent to conventionalsecuring modes between the point of origin of the spiral of Archimedesand the rotational axis of the balance. In the following description,isochronism means the working deviations as a function of variations inthe oscillation amplitude of the balance, as well as working deviationsbetween the horizontal position and the vertical positions of the watch.

In a known manner a spiral, having turns of uniform section and pitch,via a particular conformation of the inside curve and the outside curvein the plane of the spiral or most often in different planes, enablesone to obtain a concentric development of the spiral and a movement ofthe centre of the spiral weight and a variation in the spiral's inertiaduring development minimising working disruptions as a function of theamplitude and positions of the spiral with respect to the gravityvector. In addition to the fact that making such a spiral requires greatskill, the space required in height constitutes a certain drawback forits use in wristwatches that have to have, for evident aestheticalreasons, the smallest possible thickness.

For this reason, use of a flat spiral is preferred, such as that shownin FIG. 1. Such a spiral is manufactured in a known manner by windingfrom a wire or metal band of constant section over its entire length,and has a constant pitch at rest between the turns. As can be seen inFIG. 1, the inside curve is fixed, for example by laser welding, onto acollet 20, driven onto the arbour 9 of a balance 8.

With respect to this state of the art, as regards the pitch between theturns, CH Patent No. 465 537, filed in 1966, should be mentioned,wherein there is disclosed a method for manufacturing spirals of anyconfiguration, particularly with a variable pitch, from a metal strip orwire of constant section, wound in the groove of a die, then annealedand hardened. To the knowledge of the Applicant, no products of thistype have been put on the market, which leads one to assume that themanufacturing method was not, technically or economically, satisfactory.

As regards the variation in thickness of a wound metal strip, GB PatentNo. 1020 456 can be mentioned, which discloses the manufacture of amainspring by buttwelding of strips having sections that increase fromthe centre to the periphery. Such a spring is designed, with equal spacerequirement, to increase the power reserve, but it is clear that byapplying this manufacturing method to a spiral, the presence of weldswould prevent a concentric development and would not allow reproducibleisochronism to be obtained from one spiral to another.

This same principle had, moreover, already been proposed in U.S. Pat.No. 209,642 dating 1878, for improving the isochronism of a spiral madewith an inside turn of smaller section. As will be seen in the detaileddescription, experiments contradict this assertion.

The invention thus concerns a flat spiral and micro-machining orgalvanic growth manufacturing methods, for selecting the most favourableconstruction parameters in a convenient way for the purpose of improvingisochronism by the shape of the spiral as well as by the securing means.

The invention therefore concerns a flat spiral, formed of a strip madeup of a succession of turns having a pitch “p” between them, for aregulating balance mechanism, said spiral being obtained by amanufacturing method which allows almost perfect isochronism. The turnsof rectangular section are formed in a single continuous material fromthe inside curve to the outside curve, but, on certain portionscomprised between the point of attachment at the centre and the point ofattachment at the exterior, have a section “s” that is non uniformand/or one or more portions shaped outside the tracing of a perfectspiral. The expression “non uniform section” means that, for a striphaving a constant height “h”, the thickness “e” of a selected portioncan be either greater or less than the thickness of the rest of thestrip forming the spiral.

As will be explained hereinafter in the detailed description, themanufacturing method relies on micro-techniques, such asphotolithography and electroplating a metal or metal alloy, ormicro-machining a plate of thickness “h” made of an amorphous orcrystalline material such as silicon in mono-crystalline orpolycrystalline form.

According to a first embodiment, the section “s” of the turns increasesprogressively from the outside curve to the inside curve.

According to a second embodiment, which can be combined with the firstembodiment, the pitch “p” between the turns decreases regularly from theoutside curve to the inside curve.

According to yet another embodiment, it is possible to select adetermined turn portion and vary the width of the strip locally in orderto act on other parameters favourable to isochronism. This increase maybe achieved for example on the inside curve, on the outside curve or onboth curves at once, or in many other places on other portions of thespiral.

It is also possible to obtain a spiral having a turn portion thatdeviates from the curve of a perfect spiral, by having, for example, aGrossmann type inside curve.

The invention also offers the advantage of being able to manufacture atthe same time both the actual spiral and the means for securing it ontothe balance arbour, this securing means being formed by a self-lockingwasher having at the centre, for example a star-shaped contour andincluding recesses in its periphery to give it sufficient elasticity forassembly and preventing a deviation between the point of origin of thespiral of Archimedes and the rotational axis of the balance.

For a metal or metal alloy spiral, the manufacturing method basicallyconsists in applying the LIGA technique to form a mould corresponding tothe desired profile of the spiral. Given the properties of thephotoresists currently available on the market, it is possible to adjustthe thickness of the photoresist layer to obtain the entire range ofspirals with strip heights of up to several tens of a millimetre.

For a spiral made of amorphous or crystalline material, the methodbasically consists in etching a plate of said material through masks.

Other features and advantages of the present invention will appear inthe following description of different embodiment examples given by wayof non-limiting illustration with reference to the annexed drawings, inwhich:

FIG. 1 shows a sprung balance of the prior art;

FIG. 2 is an enlarged diagram of the spiral of FIG. 1;

FIG. 3A is a diagram of the isochronism obtained with the spiral shownin FIG. 2;

FIG. 3B is a diagram of the isochronism obtained with another spiral ofthe prior art;

FIG. 4 shows a first embodiment of a spiral according to the invention;

FIG. 5 is a diagram of the isochronism obtained with the spiral of FIG.4;

FIG. 6 shows a second embodiment of a spiral according to the invention;

FIG. 7 is a diagram of the isochronism obtained with the spiral of FIG.6;

FIG. 8 shows a third embodiment of a spiral according to the invention;

FIG. 9 is a diagram of the isochronism obtained with the spiral of FIG.8;

FIG. 10 shows a mode of securing a spiral according to the invention;and

FIGS. 10A to 10E show other forms for securing the spiral to the centre.

FIG. 1, which is partially torn away, shows a sprung balance of theprior art referred to in the preamble. Its features serve as a referenceto show the significant progress brought by the invention as regardsisochronism. Spiral 10 has the end of its curve at the centre 11 securedin a conventional manner onto a collet 20 driven onto the arbour 9 ofthe balance 8 pivoted between the plate 7 and the balance-cock 6. Theregulating device further includes in a known manner a balance springstud holder 5 for securing the outside curve 14 of spiral 10 and anindex 4 provided with pins 3 and an index tail 2 facing a scale 1. InFIG. 2, which is an enlarged diagram of spiral 10 alone, it can be seenthat said spiral is formed of 14 turns having a uniform rectangularsection, for example 0.05×0.30 mm from the centre curve 11 to theoutside curve 14, and that the turns have a constant pitch p betweenthem. The point of attachment of the centre curve 11 is located at adistance r from the centre of pivoting of the spiral, and that ofoutside curve 14, at a distance R, before the bend 16. In this example rand R have the respective values 0.57 mm and 2.46 mm. These values of rand R, and the number of turns, will be the same in the followingdescription, unless otherwise indicated.

With reference now to FIG. 3A, there is shown the isochronism diagram ofa spiral having the aforementioned features. The oscillation amplitudeof the balance expressed in degrees with respect to its position ofbalance is shown on the X axis The working deviation expressed inseconds per day is shown on the Y axis. This diagram includes fivecurves corresponding to the usual measurement positions with the sprungbalance, horizontal (curve 1), then vertical (curves 2 to 5, by rotationthrough 90° from one curve to the other). The dotted line corresponds tothe envelope of all the most unfavourable positions. Appreciation of theworking deviation is carried out in a conventional manner by taking intoconsideration the maximum deviation of the envelope for an amplitudecomprised between 200° and 300°. In the diagram of FIG. 3A, it can beseen that this maximum deviation, with this reference spiral of theprior art, is 4.7 seconds per day for an amplitude of 236°.

FIG. 3B shows the diagram obtained with a spiral (not shown) having thefeatures mentioned in U.S. Pat. No. 209,642 cited in the preamble,namely with a strip thickness varying between 0.046 mm for outside curve14 and 0.036 mm for inside curve 11. Contrary to what might be expectedfrom the teaching of said Patent, it will be observed that the maximumdeviation has increased to 7.7 seconds per day for an amplitude of 230°.

With reference now to FIGS. 4 and 5, there will be described a firstembodiment of a spiral the manufacture of which by micro-machining(photolithography and galvanic growth), or etching an amorphous orcrystalline material allows geometry favourable to isochronism to beobtained. As can be seen, the pitch p between one turn and the nextdecreases gradually towards the centre of the spiral. Conversely, thesection increases from the outside curve 14 to the inside curve 11.Given that the manufacturing methods give the strip a constant height,the variation in section in fact corresponds to a change in thethickness which goes from 0.036 mm for the outside curve 14 to 0.046 mmfor the inside curve 11.

In the diagram shown in FIG. 5, it can be seen that the maximumdeviation is decreased to 2.8 seconds per day for an amplitude of 242°.A favourable result could be obtained on this maximum deviation byacting solely, either on pitch p or on thickness e of the strip.

FIGS. 6 and 7 correspond to a second “Michel” type embodiment for theoutside curve 14 and for inside curve 11. The turns have a constantpitch between them and constant section corresponding to a constantthickness of 0.042 mm, with the exception of two turn portions for whichthe thickness is brought to 0.056 mm:

a portion 12 of inside curve 11 over an angular sector of approximately80° the median part of which is at substantially −110° from a referenceaxis Ox, and

a portion 15 of outside curve 14 over an angular sector of approximately20° the median part of which is at substantially +115° from referenceaxis Ox.

In the diagram shown in FIG. 7 it can be seen that the maximum deviationis no more than 1.8 seconds per day. The value of the overthickness andthe positions on the turns are given here solely by way of illustration,and it is clear that those skilled in the art can choose to have alarger number of zones of overthickness at different locations.

FIGS. 8 and 9 show a third embodiment wherein inside curve 11 is of theGrossmann type 13, i.e. having the geometry described in the work“Théorie générale de I'horlogerie” by L. Defossez. This geometry is verydifficult to obtain by deforming a metal strip. The manufacturing methodaccording to the invention however allows such a configuration to beobtained very easily without any intervention by a highly qualifiedperson. The diagram shown in FIG. 9 shows that the maximum deviation at300° is only 2.1 seconds per day.

Of course, given the freedom of configuration provided by themanufacturing methods according to the invention, it is possible tocombine the embodiments previously described to obtain a spiralaccording to the invention having improved isochronism.

FIG. 10 shows a spiral corresponding to the first embodiment (FIG. 4)wherein the collet 20 is replaced by a self-locking washer 17 formed atthe same time as spiral 10. This washer 17 has at its centre a contour19 such that it allows the arbour 9 of balance 8 to be locked withoutany play while having a certain elasticity provided by holes 18distributed about the locking contour 19 shown in a star in FIG. 10.FIGS. 10A to 10E show other possible configurations of self-lockingwasher 17 with a triangular, square, hexagonal, circular or nose-shapedlocking contour 19. When the spiral-self-locking washer assembly is madeby photolithography and galvanic growth, one can advantageously makesaid self-locking washer 17, by means of an additional step, with athickness greater than the height of the strip in order for spiral 10 tobe held better on balance arbour 9.

A spiral according to the invention made of an amorphous or crystallinematerial such as silicon can be manufactured by adapting themicro-machining methods already used for example for manufacturingintegrated circuits or acceleration meters from a silicon wafer.Reference can be made in particular to the methods disclosed in U.S.Pat. Nos. 4,571,661 and 5,576,250 concerning acceleration meters. Themethod basically consists of the following steps:

applying a silicon wafer to a substrate creating an insulating SiO₂interface;

thinning the plate to the desired strip height “h” in accordance withthe method described by C. Harendt et al. (“Wafer bonding and itsapplication to silicon-on-insulator fabrication” Technical DigestMNE'90, 2^(nd) Workshop, Berlin, November 90, p. 81-86);

forming a mask by photolithography corresponding to the desired spiralcontour;

etching the silicon wafer to the substrate, in accordance with knownmethods, such as wet method chemical etching, dry plasma etching or acombination of the two; and

separating the spiral from the substrate.

Given the very small dimensions of a spiral, it is obviously possibleand advantageous to manufacture them in batches from a single siliconwafer.

In order to manufacture a metal or metal alloy spiral according to theinvention, the LIGA method, known since the middle of the 70s is used.In a first step, the method basically consists in spreading a positiveor negative photoresist on a substrate previously coated with asacrificial layer, over a thickness corresponding to the desired stripheight “h” and forming a hollow structure corresponding to the desiredspiral contour by means of a mask by photolithography and chemicaletching. In a second step, said hollow structure is filled with a metalor a metal alloy either by electroplating as indicated for example inU.S. Pat. No. 4,661,212, or by nanoparticle compression and sintering,as indicated for example in US Patent Application No. 2001/0038803.

In a last step the spiral is released from the substrate by removing thesacrificial layer.

1. A sprung balance resonator spiral having its arbour pivoted between aplate and the balance-cock, said spiral being formed of a strip made upof a succession of turns having a pitch “p” between them, the end of theinside curve being secured to the balance arbour and the end of theoutside curve being secured to the balance-cock or to a part securedthereto, wherein the turns are formed of a single strip from the insidecurve to the outside curve and have, over certain portions comprisedbetween the point of attachment at the centre and the point ofattachment to the exterior, a rectangular section “s”, of height h andnon uniform thickness e, and/or include one or more portions shapedoutside the line of a perfect spiral.
 2. The spiral according to claim1, wherein the section “s” of the turns increases regularly from theoutside curve towards the inside curve.
 3. The spiral according to claim1, wherein the pitch “p” between the turns decreases regularly from theoutside curve towards the inside curve.
 4. The spiral according to claim1, wherein the section “s” of the turns increases and the pitch “p”decreases from the outside curve towards the inside curve.
 5. The spiralaccording to claim 1, wherein a portion of the inside curve has a largersection than that of the strip forming all of the other turns.
 6. Thespiral according to claim 1, wherein a portion of the outside curve hasa larger section than that of the strip forming all of the other turns.7. The spiral according to claim 1, wherein a portion of the insidecurve and a portion of the outside curve have a larger section than thatof the strip forming all of the other turns.
 8. The spiral according toclaim 1, wherein the inside curve has a Grossmann type configuration. 9.The spiral according to claim 1, wherein the inside curve is extended bya self-locking washer formed at the same time as the strip and acting asa collet to position said spiral on the arbour of the balance, thusallowing the distance and orientation of the point of origin of thespiral of Archimedes to be controlled with respect to the rotationalaxis of the balance.
 10. The spiral according to claim 9, wherein theself-locking washer has a thickness greater than the height “h” of thestrip.
 11. A method for manufacturing a spiral from a plate of amorphousor crystalline material, said spiral being formed of a single strip ofrectangular section having a non uniform thickness e and/or comprisingone or more turn portions shaped outside the line of a perfect spiral,wherein it consists in etching said plate along the desired contour ofthe spiral by means of a mask.
 12. A method for manufacturing a metal ormetal alloy spiral formed of a single strip of rectangular sectionhaving a non uniform thickness e and/or comprising one or more turnportions shaped outside the line of a perfect spiral, wherein a mould isformed by the LIGA method corresponding to the desired contour of thespiral, and that the metal or alloy is added to said mould.
 13. Themanufacturing method according to claim 12, wherein the metal or alloyis added by electroplating.
 14. A manufacturing method according toclaim 12, wherein the metal or alloy is added in the form ofnanoparticle powder that is compressed, and then sintered.